The vapor-liquid equilibrium of a formaldehyde-water system forms an azeotrope at a formaldehyde concentration of about 22% by weight under normal pressure. Accordingly, in general, when crude formaldehyde containing water and methanol and having a formaldehyde concentration above the azeotropic point is subjected to distillation, the formaldehyde in the liquid phase is concentrated such that paraformaldehyde, i.e., a low polymer form of formaldehyde, is ultimately precipitated. It has been therefore difficult to obtain formaldehyde having high purity by distillation of crude formaldehyde containing water and methanol.
It has been proposed to react the crude formaldehyde containing water and methanol with a higher alcohol to once form a hemiacetal, which is then dehydrated and pyrolyzed to obtain formaldehyde having a decreased water content (U.S Pat. No. 2,848,500). This technique, however, requires complicated steps and involves side reactions or denaturing. In addition, the purity of the resulting formaldehyde is unstable Hence, it has been necessary to combine this process with the purification process of a formaldehyde gas as hereinafter described before high purity formaldehyde can be obtained.
U.S. Pat. No. 2,678,905 discloses a technique for purifying a formaldehyde aqueous solution by extractive distillation. This technique aims at separating organic compounds contained in the formaldehyde aqueous solution, not water That is, the technique does not relate to the production of high purity formaldehyde by extractive distillation while avoiding azeotropy of formaldehyde with water.
Further, U.S. Pat. No. 2,780,652 discloses a process for purifying formaldehyde gas, in which formaldehyde gas having a concentration of at least 95% by weight is brought into contact with polyethylene glycol dimethyl ether in a counter-flow system. According to this process, impurities, such as water, present in 95 wt % or higher formaldehyde gas, can be absorbed in polyethylene glycol dimethyl ether to thereby obtain high purity formaldehyde gas. However, as mentioned in the patent, the process would bring no economical profit unless the starting formaldehyde has a concentration of 95 wt % or more. If the concentration is less than 95 wt %, the ratio of formaldehyde absorbed into the polyethylene glycol dimethyl ether to purify formaldehyde becomes very high.
Hence, none of the above-described conventional processes relate to the production of high purity formaldehyde directly through extractive distillation of crude formaldehyde containing water and methanol, which is produced on an industrial scale.